Before I discovered the Opacity Mask in Illustrator, I thought that it was only possible to do complex transparencies in Photoshop. However, it is actually pretty simple to acheive many of the same effects in Illustrator using the Opacity Masking feature in the Transparency Panel.

For instance, in order to create a text reflection in Illustrator, simply type the desired text on a fairly dark background.

Then duplicate the text, right click to select Transform>Reflect>Horizontal, and line this up underneath the original text.

Now you need to create a simple rectangular black and white gradient and cover up the text on the bottom. The blacks will become transparent and the whites become opaque once you apply the mask, so you can experiment with the gradient settings to get things just right for your purposes. There is an option on the Transparency Panel to invert the mask as well, in case you would rather have the blacks opaque and the whites transparent.

Make sure to select both the text and the gradient you just created, taking care not to select anything else like the background. You can select the background and press CTRL+2 to lock it down if you are having trouble selecting items on top of it. Simply CTRL+ALT+2 to unlock it when you need to select the background again. Open the Transparency Panel, and select Make Opacity Mask from the Transparency Panel Options Menu.

Adjust the general opacity to taste. Also, notice that the Transparency Panel has an option for you to select the masked object or the mask itself and work with each of these items individually if necessary - just click on the left or right box to select it.

The rest is up to you. I decided to adjust the background gradient to create more of a horizon line, placing the darkest background colors towards the bottom.